Aircraft manufacturers are under constant pressure to improve the fuel efficiency of modern commercial transport aircraft. Improved fuel efficiency can increase the range of the aircraft, reduce CO2 emissions and/or reduce the cost of operating the aircraft. While modern, high-bypass turbofan engines have shown significant improvements in fuel efficiency when compared with the turbojet engines developed at the beginning of the jet age, aircraft manufacturers must continually strive to improve aircraft and aircraft engine performance in response to customer demands.
One relatively recent development in modern commercial aircraft includes replacing hydraulic and/or pneumatic aircraft actuators with electrically powered actuators. These actuators may be used to power a myriad of aircraft systems, including flaps, ailerons, and rudders. Electrically powered actuators use power provided by generators that are in turn driven by the aircraft turbofan engines (which also provide the main propulsive force for the aircraft). While this technology evolution has proven beneficial, it increases the power demands placed on aircraft engine generators, which typically requires an increase to the size of the generators. Furthermore, current aircraft engine generators are typically less than 45% efficient at converting Jet-A aviation fuel into electrical power during cruise operations. Accordingly, there is a strong desire to improve the efficiency with which electrical power is generated on board the aircraft, so as to keep the engine size as low as possible, reduce the amount of fuel carried aboard the aircraft, and/or improve the overall efficiency and environmental performance of the aircraft.
One approach to improving the efficiency with which electrical energy is generated onboard the aircraft is to use electrochemical fuel cells. For example, fuel cells have been identified as a replacement for the aircraft auxiliary power unit. However, fuel cells tend to be heavy, in many instances due to the peripheral equipment (e.g., compressors) used to provide air to the fuel cells for operation.
Another pressure that aircraft manufacturers face is reducing the emissions of potentially harmful gases present in the exhaust stream from the turbofan engines. Such emissions typically include NOx emissions, which can pollute the air near airports, and can lead to the formation of ozone at cruise altitudes. In response to pressures to reduce the emissions of such gases, low NOx combustors have been developed. These combustors typically operate at lower peak temperatures than more conventional combustors, by using a fuel-lean mixture, and by significantly increasing the degree to which the fuel is mixed with air before being combusted. However, a potential drawback with this arrangement is that the lean mixture may produce an unstable flame. As a result, the flame may be more likely to blow out (“flameout”), which can produce an unplanned unstart of the aircraft engine. One approach to addressing this drawback is to provide the combustor with a small fuel-rich spray at each nozzle. However, burning such a spray tends to produce the very emissions that the low NOx combustor is intended to reduce. In light of the foregoing, there is a desire to both improve the overall fuel efficiency and robustness of aircraft engines and reduce the emissions of potentially harmful exhaust products.